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Laminated Busbars for EV Battery Systems – Thermal Management

10/10

2026

By A&JLINK Engineering Team · Last updated October 2026 · 8 min read
The short answer
Why the busbar is both a heat source and a heat path in an EV battery pack.
  • ✓It generates heat. Its own resistance and its joint resistance produce heat the pack has to remove — and at several hundred amps, that loss is not small.
  • ✓It moves heat, too. The same conductive geometry that cools the pack can carry a failed cell's heat into its neighbours.
  • ✓So it is a safety component. Low-resistance joints, cooling integration, and propagation protection are what make a battery busbar safe.
An EV battery pack is thousands of cells grouped into modules, modules grouped into a pack, and the pack wired to the inverter. The conductors doing that wiring — the busbars — carry the pack's full current. In most packs those conductors are laminated bus bars — stacks of flat conductive layers separated by thin dielectric film, chosen to pack high current into a small, controlled shape. Current in any conductor means heat, and heat in a battery pack is not a neutral thing: it shortens life, cuts performance, and at the extreme is the mechanism behind battery fires.
Scope: this article covers busbar thermal management in EV battery systems — where the heat comes from, why the joint is the weak point, how heat is removed, how thermal-runaway propagation is prevented, and how thermal performance is verified before production.

1. Why Thermal Management Defines an EV Battery Busbar

Lithium-ion cells are comfortable in a narrow band. Below about 0 °C they accept charge slowly and can plate lithium; above roughly 40 °C their capacity fades faster and their internal resistance climbs, both of which shorten the pack's life. Push a cell past about 130 °C and the separator can begin to fail, which is where thermal runaway starts. Keeping every cell inside the comfortable band is the pack's main thermal job.
The busbar sits inside that job in two ways. It carries the high current — several hundred amps in normal driving, and a sustained high current during fast charging — so it generates heat of its own. And because it physically connects the cells, it also conducts heat between them, which can be good or bad depending on the design.
A&JLINK supplies laminated busbar for EV assemblies for both traction inverters and battery packs, engineered around those current and thermal conditions. In a battery pack, the busbar is not just a connector; busbar thermal management is part of the pack's core safety and life design.

2. Where the Heat Comes From

Busbar heat has two sources, and they scale differently. The first is the conductor itself: every conductor has resistance, and the loss is I²R, so it grows with the square of current. At a pack current of 300–400 A, even a few tens of micro-ohms of conductor resistance produces real watts along a busbar, and those watts have to leave the pack.
The second source is the joints. A busbar connects to cells, module terminals, and contactors through joints, and every joint adds contact resistance on top of the conductor's own. Contact resistance is often the larger of the two at a poorly made joint, and unlike the conductor it tends to grow over life as the joint cycles. The physics of how temperature interacts with a busbar's own performance are covered in our guide on how temperature affects laminated busbar performance.
The practical point: the total heat a busbar puts into the pack is set by both its bulk resistance and its joints. Reducing one while ignoring the other does not solve the thermal problem.

3. The Joint Is the Hotspot

In service, the hottest point on a battery busbar is almost always a joint, not the middle of the conductor. That is because a joint concentrates resistance into a small area, and because joints are also where mechanical degradation first appears.
A bolted joint relies on contact pressure to keep resistance low. Under the charge–discharge thermal cycling of a battery pack, the parts expand and contract at slightly different rates, and over thousands of cycles a bolted joint can relax — the pressure drops, the resistance rises, and the joint runs hotter, which accelerates the relaxation further. That feedback is how a joint becomes a hot spot and eventually a failure.
Welded or integral joints — the kind a laminated busbar uses — avoid the moving fastener entirely. There is no torque to relax and no contact surface to corrode, so the joint's resistance stays stable across the pack's life. For a battery application where maintenance means a pack teardown, that stability is the whole point.

4. Getting Heat Out: Cooling the Busbar

Because a busbar is metal, it is also a heat path. Laminated bus bars present a wide, flat surface rather than a round cable bundle, which spreads the current and the loss over a large area and gives heat a clean route out of the conductor.
In many packs, the busbar is deliberately coupled to the thermal management system. A cold plate or liquid-cooling loop runs under or beside the modules, and the busbar can be thermally tied to it through the module structure, so the heat the busbar generates is carried away with the cell heat rather than left to accumulate. Where the busbar is not actively cooled, its cross-section and material are chosen to keep the temperature rise acceptable by passive means — more copper lowers loss but adds mass, while aluminum saves weight at the cost of higher resistance.
There is a trade-off hidden here. The same flat, conductive geometry that makes the busbar a good heat sink also makes it a good bridge between adjacent cells. That is fine when the goal is to move heat out, and dangerous when the heat is coming from a cell that has failed — which is the subject of the next section.

5. Thermal Runaway and Propagation – the Safety Case

The worst thermal event in a battery pack is thermal runaway: a cell that generates heat internally faster than it can be removed, driving its own temperature up until it vents and burns. At that point, the question becomes whether the failure stays in one cell or spreads to its neighbours — and the busbar is one of the paths by which it can spread.
Because the busbar connects many cells electrically, it also connects them thermally. A runaway cell's heat can travel along the conductor into the next cell, raising it toward its own runaway threshold and starting a chain reaction. Preventing that propagation is a safety requirement, and it shapes the busbar design directly.
The countermeasures are structural. Thermal barriers and potting between cells slow heat transfer through the pack. Fuse elements or narrowed sections in the busbar can open the connection to a failed cell and cut off both the current and part of the heat path. And the busbar's geometry can be designed so that a single cell's failure does not have a wide, low-resistance thermal route into its neighbour. A battery busbar is specified as much for what it does in a failure as for what it does in normal operation.

6. Verifying Thermal Performance

You cannot wait for a pack to fail to find out whether its thermal design is adequate, so thermal performance is verified by test and simulation before production.
On the test side, a temperature-rise test runs the busbar at rated and peak current and measures the hottest point — usually a joint — to confirm it stays within the insulation's temperature class and the pack's limits. A thermal-cycling test repeats the pack through its operating temperature range thousands of times to catch joint relaxation and insulation fatigue before they appear in the field. On the design side, thermal behavior is predicted before hardware exists; our guide on laminated busbar thermal simulation covers how that is done and why it matters for catching hot spots early.
Together, test and simulation form the evidence that the busbar will hold its temperature across the pack's life — which is what a safety-critical component needs to demonstrate. This evidence is what separates a busbar thermal management approach that is designed from one that is only assumed.

7. What to Specify for EV Battery Busbar Thermal Performance

When you specify a busbar for an EV battery pack, the thermal items below decide whether it performs in service:
  • Rated current: continuous and peak, including fast-charge and acceleration demand.
  • Allowable temperature rise: the maximum the joint and conductor may reach at the worst-case pack ambient.
  • Contact resistance: specified at the joint, together with the joint type (welded or integral preferred).
  • Material and cross-section: copper or aluminum, sized to the current and to the cooling available.
  • Insulation: a temperature class with thermal endurance assessed per IEC 60216, and insulation coordinated to the pack's voltage per IEC 60664-1.
  • Cooling interface: how the busbar couples to the pack's thermal management.
Because no two packs are laid out identically, most battery busbars are customized parts. Involving the supplier early in the mechanical and thermal design shortens the path to a component that fits and cools correctly the first time.

8. Frequently Asked Questions

Why does a battery busbar get hot?
Two reasons: the resistance of the conductor itself (I²R loss, which grows with current), and the contact resistance at its joints. At pack currents of several hundred amps, both produce meaningful heat.
What is the hottest part of a busbar?
Usually a joint. Contact resistance concentrates loss into a small area, and joints are also where thermal cycling causes the most degradation.
Can a busbar cause thermal runaway to spread?
It can act as a thermal bridge between cells, so a failed cell's heat can travel along it. Propagation is prevented by thermal barriers, fuse elements, and geometry that limits the cross-cell heat path.
How is battery busbar thermal performance verified?
By temperature-rise testing at rated current, thermal-cycling testing, and simulation during design — run before production, not discovered in the field.

9. Summary

In an EV battery pack, the busbar is both a source of heat and a path for it. Its own resistance and its joints generate heat that must be removed; its high-conductivity geometry can either help cool the pack or, if a cell fails, help a failure spread. Managing that means keeping contact resistance low with welded or integral joints, tying the busbar into the pack's cooling, and designing geometry and protection so that a single cell fault stays a single cell fault. Busbar thermal management is what turns a connector into a safety feature. Talk to our engineers about your pack's current, temperature, and safety requirements.
Specifying a Busbar for an EV Battery Pack?
Send us your pack current, temperature limits, and cooling approach — our engineers will review feasibility and come back with design recommendations.
Talk to an Engineer

References & Standards

  1. IEC 60664-1 — Insulation coordination for equipment within low-voltage supply systems. International Electrotechnical Commission. webstore.iec.ch
  2. IEC 60216 — Electrical insulating materials – Thermal endurance properties. International Electrotechnical Commission. webstore.iec.ch

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